“They’ve been having issues for a while now,” says a source close to the couple. “They tried to work through them, but it didn’t work. They’re still committed to being good parents to their children.”

There is no third party involved in the relationship, the source says.

Letourneau’s friend and former attorney David Gehrke says it was Fualaau who filed for separation.

“She could try to contest it, but that’s legally tough to do,” he says. “Perhaps if they want to figure out about the children and child support, but their kids are getting pretty old now.”

“When one person is unhappy in the relationship, the other person can’t be happy either,” he continues. “And there are two people in this relationship.”

Their initial affair — which began while Letourneau was a 34-year-old mother of four and Fualaau was 12 — ignited national controversy and revulsion, particularly after she defied a court order to stay away from Fualaau. “The two of them want to be together,” a friend of Letourneau’s told PEOPLE in a 1998 cover story. “They’re drawn together like magnets.”

Prosecutors took a different view, arguing at the time that Letourneau “doesn’t believe she did anything wrong.”

After the relationship was revealed, she pleaded guilty to two counts of second-degree child rape and was sentenced to seven and a half years in prison.

“I think, What would my life have been like if I had never made a move on Mary?” he said. “What if I had kept it as a crush and left it at that? Where would I be and where would she be — what would life be like?”

But he said he didn’t see an answer to such speculation. “I can never see more than the question.”

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Vili Fualaau in October 1998

John Froschauer/AP

Vili Fualaau and Mary Kay Letourneau (center left and center right) with their two daughters, Audrey (far left) and Georgia (far right), in 2015

Heidi Gutman/ABC/Getty

Nearly a decade later, in 2015, Fualaau and Letourneau sat down for an interview with ABC’s Barbara Walters, where they discussed their relationship, marriage and parenting.

Letourneau revealed then that she would like to get her name taken off the sex offender registry, though officials in Washington state tell PEOPLE she is still a registered offender.

“There is a story of us that has a life of its own, but it’s not our story,” she told Walters.

When Walters asked Fualaau how he’d feel if the girls came home and said they were sleeping with their teacher, he responded, “I don’t support younger kids being married or having a relationship with someone older. I don’t support it.”